Expansion chamber

[antithesis]

Now I have no real plans or ways to implement this idea, but I want to understand the underlying physics better..anyhow...

I was thinking back to when I worked in heating and air conditioning, and at the same time about air guns (of course). I had started considering the concept of the expansion tank in a closed loop boiler system, and had started Wondering if a similar concept could be used to any effect in maintaining pressure during shot cycle, making better use of systems with limited valve volume, raising average pressure during the shot.

This would be some form of pre pressurized space, isolated from the actual air supply, able to expand as pressure is replenished in the plenum, perhaps something like a small air ram or even something heavily spring loaded.

I'm sure there is a physical concept I am overlooking here but I keep thinking about this and need someone more knowledgeable than I to burst my bubble lol... here is looking at you Bob😁

[rsterne]

I believe there was such a system in the Webley Paradigm.... IIRC it used Belleville washers to help maintain pressure during the shot.... AFAIK, it never made it to production....

Bob

[antithesis]

I believe there was such a system in the Webley Paradigm.... IIRC it used Belleville washers to help maintain pressure during the shot.... AFAIK, it never made it to production....

Bob

Webley paradigm you say? Well looks like I got a lil digging to do.

I just wanted to see if my thinking was in line with the applicable physics, I'm sure that it is difficult to properly implement but I think it could hold alot of promise in systems like mpp and the like..thanks Bob 😁

[rsterne]

We have considered a couple of ideas in the past, using a piston to either maintain the pressure during the shot, or to even boost (double) the pressure to increase FPE.... Most of the designs end up with the piston crashing into the end of the cylinder, and I think they are likely to self-destruct in short order....

Bob

[Techie]

This post got me thinking about a hybrid spring-piston/PCP airgun.  Maybe a small piston with a short stroke and low mass to reduce the recoil/movement of a springer design, but boost the power of the PCP.  I guess it's a solution looking for a problem since PCPs already produce as much power as accuracy allows.  Still, a design like this could up the shot count of a PCP.  Timing would have to be perfect and it's a complex design, but still might be fun to experiment with.

[rsterne]

Mike, are you thinking along the lines of using a spring loaded piston to compress the HPA in a PCP, to raise the temperature and pressure like in a Springer?.... If so, consider the force on such a piston.... If it was only 1/2" in diameter and the HPA was at only 2000 psi, the force on the piston would be 393 lbs.... To double the pressure, that would also double to 785 lbs.... If you had a 2" long cylinder with a 1" piston stroke, the spring energy required would be 49 FPE.... to compress 6.4 cc to half that volume.... 

Actually, since the pressure would increase even more, due to the adiabatic compression, even more energy would be required to move that 1/2" piston a distance of 1".... but the resulting pressure would be higher, too....

Bob

[Techie]

I was thinking of adding the air already compressed by the piston to the HPA stream.  Timing would be everything.  It's probably a totally stupid idea because I don't understand the physics, nor can I figure out how you would devise a system to get this timing correct.  Because of you, I now appreciate the forces involved and how the HPA could force the piston backwards unless the air port from the piston were closed off at the perfect time.

[antithesis]

Like I said, I was quite sure the design and implementation would be a whole different beast (theory vs practice)..

Though for my two cents worth, I was considering a mpp that would be getting pressurized against a strut that would preferably be adjustable pressure, the resting valve volume nearing zero, the pumps being all similar in effort, pushing the face back as the valve pressure exceeded the ram pressure...oh well a man can dream, and it doesn't seem drastically different from what I took away looking at the design of the paradigm

[rsterne]

Bill, that is a very clear description of your original thoughts.... makes it easy to understand.... I think the problem is that on firing the piston / valve face would crash into the end of the chamber.... The concept of constant pressure while the volume is variable is intriguing, however....

Bob

[Insanity]

One way to stop a crash is to use a suitable gas piston and lock it in so its max length will not encroach on the valve. Something similar with a spring but with some sort of a theater.

[Rob M]

i had an idea , havent yet built it.. It was basically a double sided piston.. on the large piston side, co2 fires into the chamber driving the piston violently torward the high pressure side.. In the high pressure side, air could be atmospheric.. the swept volume could be medium springer range , and this could capitalize on adiabatic heating on the atmospheric air side. hope that makes sense, its a lazy mans booster system functioning in rapid violence. PRimary question is always , why not just use the co2 to fire the proectile ? again , the adiabatic compression

[rsterne]

Matt, haven't you just replaced the "piston crash" with a crash inside the gas ram?.... It's possible that with careful sizing of the compression ratio, volume, and transfer port size it could be made to work like a Springer, with the piston bouncing off the air itself.... but doing that with an MPP where you are dealing with different pressures depending on the number of pumps would see to be.... challenging....

Rob, I can't see how adding a stage could improve the performance.... as the system is still being driven by an expanding (and cooling) gas.... Usually increasing the complication reduces the efficiency.... but I could be wrong, and willing to say so if somebody shows an improvement in performance with such a system....

My first thought is that if the "pressure" side is similar to a medium Springer (which can reach 1500-2000 psi on a 1" piston with a 4" stroke).... you would need to drive it with a huge cylinder for the 850 psi (and dropping as it expands) of the CO2 to provide enough force to compress the air to that value in the very short period of time required.... Let's say we needed 1700 psi on the HPA side, without cooling the CO2 side would need to be 1.414" diameter, with a 4" stroke.... That is 103 cc of CO2 gas at 850 psi.... If my math is correct, that is greater than the entire contents of a 12 gr. cartridge.... even if you could convert it instantly to vapour without a temperature drop.... 

Bob

[antithesis]

Aside from Webley going belly up, I can begin to see why this project was ultimately shelved ..though a hot tuned version of the prototype I saw could make for one killer FT gun...it would be nice to get some real-time input from the designer (who still keeps the basic info on his web page)

[Insanity]

What I am thinking I can see in my head but right now with it being late and it being a long sunday I cannot put thought to words so tomorrow hen i have a few at work ill try to convey what I am thinking.

[Rob M]

Matt, haven't you just replaced the "piston crash" with a crash inside the gas ram?.... It's possible that with careful sizing of the compression ratio, volume, and transfer port size it could be made to work like a Springer, with the piston bouncing off the air itself.... but doing that with an MPP where you are dealing with different pressures depending on the number of pumps would see to be.... challenging....

Rob, I can't see how adding a stage could improve the performance.... as the system is still being driven by an expanding (and cooling) gas.... Usually increasing the complication reduces the efficiency.... but I could be wrong, and willing to say so if somebody shows an improvement in performance with such a system....

My first thought is that if the "pressure" side is similar to a medium Springer (which can reach 1500-2000 psi on a 1" piston with a 4" stroke).... you would need to drive it with a huge cylinder for the 850 psi (and dropping as it expands) of the CO2 to provide enough force to compress the air to that value in the very short period of time required.... Let's say we needed 1700 psi on the HPA side, without cooling the CO2 side would need to be 1.414" diameter, with a 4" stroke.... That is 103 cc of CO2 gas at 850 psi.... If my math is correct, that is greater than the entire contents of a 12 gr. cartridge.... even if you could convert it instantly to vapour without a temperature drop.... 

Bob

and thats the downfall.. I consulted someone far more knowledegable than myself , and to make it work you would need a massive amount of co2( infinite in theory) to follow the piston the full stroke , otherwise its a complete loss ..( but there would be more power on the air side than the co2 side , if only a  mediocre percentage

[rsterne]

but there would be more power on the air side than the co2 side , if only a  mediocre percentage

ummmmmmmmmm…. something from nothing?.... perpetual motion is implied....   

Bob

[Rob M]

but there would be more power on the air side than the co2 side , if only a  mediocre percentage

ummmmmmmmmm…. something from nothing?.... perpetual motion is implied....   

Bob

well no , the air side would undergo adiabatic heating , the co2 side is essentially acting as a very powerful gas ram.. SO for a given caliber, thered be more energy on the air side because peak pressures would in fact be higher

[Rob M]

total energy input and output is different.. actual fpe at the pellet thered be a gain/ matter off fact , the total energy input would be higher in this case.. I hope im clarifying that if the pellets max theoretical speed on co2 would be 500 fps on co2 , the adiabatic heating ( and hence higher pressures might yield 600 fps ) by action of the peak pressure.

[antithesis]

but there would be more power on the air side than the co2 side , if only a  mediocre percentage

ummmmmmmmmm…. something from nothing?.... perpetual motion is implied....   

Bob

Perhaps, loosely speaking, he meant in terms of the dynamic advantage of air being the better flowing medium? I'm stretching here and won't speak for anybody else 😒

That is perhaps irrelevant to the point being discussed, but hypothetically speaking id rather have a given pressure charge of air than co2 (that is if it kept pressure through the cycle like co2 tends to due to boiling off, but I digress)

[Rob M]

but there would be more power on the air side than the co2 side , if only a  mediocre percentage

ummmmmmmmmm…. something from nothing?.... perpetual motion is implied....   

Bob

Perhaps, loosely speaking, he meant in terms of the dynamic advantage of air being the better flowing medium? I'm stretching here and won't speak for anybody else 😒

That is perhaps irrelevant to the point being discussed, but hypothetically speaking id rather have a given pressure charge of air than co2 (that is if it kept pressure through the cycle like co2 tends to due to boiling off, but I digress)

and yes, thats another factor at play , even if we didnt have the heating of the compression cycle.